1. Field of the Invention
The instant invention relates to a device for measuring changes in the vertical distance between the roof and floor of a mine.
2. Background of the Prior Art
Many devices have been utilized to measure the distance between the floor and the roof of a mine for the purpose of determining whether the roof is subsiding.
U.S. Pat. No. 3,092,912, for example, discloses a tape extensometer for measuring roof subsidence. This device includes a variable length roof tape which is secured at one end to a reel and the other end to the mine roof. A floor tape has one end secured to the mine floor and the other end passes over a pulley which is rigidly connected to the roof tape reel and is anchored by a counter weight. This device enables one to determine changes in the vertical distance as the counter weight which anchors the floor tape over the pulley moves. This device requires the user to attach the tape to the mine roof each time he wishes to make a measurement. This is impractical when the mine roof is very high.
U.S. Pat. No. 1,427,773 discloses a deflection indicator for measuring the deflection of beams and floors under loads This device relies on vertically installed poles or bars which move past each other to indicate movement of the floor and/or the roof. The requirement of a rigid pole or bar would make this device less than useful in a mine environment. For example, since the distance between the floor and the roof of a mine may be 60 feet or more, spanning this length with poles or bars would be difficult and moreover the use of poles or bars of this length would make the device more expensive than need be.
Other references which disclose the use of extensometers include U.S. Pat. Nos. 3,646,553; 3,885,423 and 4,001,942. These references disclose extensometers which are anchored directly in the mine wall or roof. Thus, the movement measured by the individual extensometers is movement of the surface relative to the under or overlying bulk of the mine floor or roof.
Other devices are known which may be used for measuring linear distances which utilize gages that may be separated from the work that is being measured and reinstalled at a later date to determine any change in the measured distances. See for example, U.S. Pat. No. 2,445,340. In a mine the measurement of changes in linear distance is not important for predicting roof collapse.
One reference which utilizes telescoping cylindrical members to indicate earth movement is found in U.S. Pat. No. 3,786,503. This reference, like the deflection indicator described above discloses a device which requires that the device itself be sized to engage both the roof and floor of a mine hole. For this reason, this device would not be suitable for use in mines having a vertical distance between the floor and roof of up to 60 feet or more. Moreover, the device described in U.S. Pat. No. 3,786,503 is intended to electrically signal only when the roof has subsided a predetermined distance along the vertical. There is no indication that this device might be used to measure subsidence on a continuous basis.
It is clear from the above that it would be desirable to have a relatively simple device which is capable of measuring the changes in the vertical distance between the roof and the floor of a mine with sufficient accuracy to enable a mine engineer to determine whether there is danger of roof collapse.
Therefore, it is one object of this invention to provide a device which can be easily installed in a mine to measure the change in the vertical distance between the mine floor and the roof.
It is another object of this invention to provide a device which is portable and can be moved between various points to measure the change in the vertical distance between the floor and roof of a mine.
Furthermore, it is another object of this invention to provide a device which can be installed in place with a minimum effort.
It is another object of this invention to provide a device which is compact and can be utilized in conjunction with easily obtained materials such as anchor bolts, steel cable, etc. to measure the subsidence of the roof of a mine.
Other objects and advantages of the instant invention will become apparent from a careful reading of the specification below.